A number of methods and devices have been proposed for measuring the speed of objects such as baseballs and tennis balls and projectiles such as arrows and bullets. One class of such methods and devices uses a time-distance measurement in which two positions of the moving object are defined, and the times at which the object is present at each position is measured, the elapsed time of the travel of the object between the two positions is computed from the time measurements, and the known distance between the positions is divided by the elapsed time to calculate the speed of the object. The devices for such measurements typically require multiple optical or other sensors. Such methods and systems can produce valid speed measurements, but the cost or complexity of device design, setup and use can present disadvantages to the user.
Another class of speed measurement devices uses continuous wave (CW) Doppler radar technology. Devices in this class use reflected waves, sometimes sonic in nature, but frequently radio frequency electromagnetic radiation (RF). RF systems can be used to detect moving objects by illuminating the object with the electromagnetic field of the radar and producing an electrical signal at a Doppler frequency which is a measure of the relative speed of the moving object. This technology has been pioneered and developed by the defense industry in the United States, is well documented in textbooks and reports, and has found numerous applications in consumer products. Security motion sensors, industrial position sensors and police radar units are examples of current uses of Doppler radar systems.
Doppler radar has been used in sports applications to measure the velocities of sports objects or players relative to one another or relative to a reference point. Examples of sports radar in use are found in U.S. Pat. No. 4,276,548 to Lutz and U.S. Pat. No. 5,199,705 to Jenkins et al. Conventional sports radar includes xe2x80x9cspeed gunsxe2x80x9d for measuring baseball or softball speed, such as disclosed in the Lutz patent. Available sports radar units generally occupy considerable volume, for example approximately 200 cubic inches, which requires that they be maintained stationary when used. Further, such systems may cost several hundred dollars. These units are typically operated by a third person somewhat remote from the players or the objects being measured.
Implementation of prior art CW Doppler radar systems is relatively complex, generally involving the use of an RF oscillator and signal generator, an antenna system to radiate the oscillator output into free-space and to receive a portion of the transmitted electromagnetic energy that is reflected by the moving object, a transmit/receive switch, diplexer, or circulator device if a single antenna is used for both transmit and receive rather than separate transmit and receive antennas, and various local oscillators, mixers, phase-locked-loops and other front-end circuits to heterodyne, demodulate and detect the Doppler signal. This complexity imposes high cost and size requirements on the radar units, which have heretofore discouraged the utilization of CW Doppler technology in consumer applications where extremely small size and low cost are necessary for practical end product realization.
In electronics applications unrelated to those discussed above, Doppler radar systems using simple homodyne circuits have been known. Such applications include defense applications such as ordnance proximity fuzes and target detectors where Doppler modulation provides evidence of a target encounter. Validation of the presence of target signals within a prescribed Doppler frequency passband, and the detection of amplitude build-up as the target encounter distance decreases, are sufficient for signal processing and decision making in such systems, obviating the need to accurately measure or calculate the specific velocity magnitude or speed. For example, for general proximity sensing applications, mere detection of an increasing distance signal is satisfactory. However, applications requiring a speed measurement necessitate determination of the specific Doppler frequency and a calculation of a corresponding speed value. Such homodyne circuits are but among hundreds or thousands of circuits and modulation schemes that in some way carry speed information but which have not been considered practical for providing speed measurements. Accordingly, circuits of a size or cost that are practical for consumer applications such as sports object speed measurement have not been known or available.
Existing Doppler speed measuring devices suffer from loss of accuracy due to the inability to place the unit in or close to the path of the moving object, resulting in a reduction in the speed measurement to the cosine of the angle between the object""s velocity vector and the line of the Doppler signal between the unit and the moving object. Further, the Doppler units must be positioned where they are not subjected to damage by collision with the object.
Accordingly, a need exists for a low cost, effective, small size, low power device useful for measuring and displaying or otherwise outputting the speed of objects in consumer applications such as sports and sports training.
A primary objective of the present invention is to provide a small size, low cost, low power device for measuring object speed that is practical for consumer applications, particularly recreation and sports. It is a particular objective of the present invention to provide a sports radar unit for measuring and outputting the velocity magnitude or speed of a sports object or projectile being propelled or shot by a user or from some form of launching implement. It is a more particular objective of the invention to provide such a speed measurement apparatus and method for measurement of the speed of balls, bullets, markers, paintballs, arrows, and other objects being shot, thrown or launched by a user or from a gun, bow or other launcher or similar implement.
According to some embodiments of the present invention, there is provided a radar speed sensor that is small in size, low in cost, low in power consumption and radiated energy, that measures and outputs the speed of an object. The sensor also may display the measured speed to a user. Further according to other embodiments of the invention, a radar unit is provided that is adapted for mounting at or near the path or point of reception of the moving object, particularly at the location of, or on, the implement or person from which the object is being shot or otherwise launched. The unit produces a radar speed measurement and produces an output signal that can operate a display or other device that interprets the speed measurement.
The positioning of the speed measuring unit is such as to facilitate the use of a low power, short range signal and accurate speed measurement. In the illustrated embodiments, the unit employs continuous-wave Doppler radar and transmits and receives RF energy in a microwave frequency range, for example, a frequency of approximately 2.4 GHz or 5.8 GHz or higher. Frequencies in the 10-25 GHz range can, for example, be used. A frequency of 5.8 GHz, for example, is suitable for a number of sports applications. Governmental regulations restrict the available frequencies differently in different countries. The frequency is 10.5 GHz may be required in some countries, and the 10.5 GHz frequency, which is available in most countries, is useful where narrow-beam low-power radiation is desired.
The speed measuring unit according to one embodiment of the invention, includes a radar transmitter and receiver that employs a single simple CW Doppler homodyne circuit preferably having an oscillator-detector that is based on a single transistor, which utilizes resonant circuit elements of the oscillator as an antenna to radiate energy into free-space. A portion of the radiated energy strikes the nearby moving object and is reflected back to the oscillator-antenna circuit where it is mixed with the oscillator signal. The coherent relationship of the transmitted and received signals in a simple homodyne circuit produces a Doppler frequency modulation as the distance to the moving object changes.
The illustrated embodiments of the present invention make use of the phenomena whereby, at a given separation distance between the radar and the moving object, the received object-reflected signal is exactly in-phase with, and reinforces, the oscillator signal, but as the separation distance changes by each one-quarter wavelength of the transmitted signal, the total two-way travel distance to the object and back changes by one-half wavelength, resulting in an out-of-phase or canceling relationship between the received and transmitted signals. Each distance change of one-half wavelength results in a two-way radar round trip change of one wavelength, thus producing one complete cycle of modulation. As the distance to the moving object changes by successive one-half wavelength increments, multiple cycles of modulation are produced. The frequency of this modulating signal is the Doppler frequency, which is equal to the velocity of the moving object expressed in terms of one-half wavelengths of the transmitted signal as follows:       f    D    =            v                        λ          t                /        2              =                  2        ⁢                  vf          t                    c      
where:
ƒD is the frequency of Doppler modulation,
xcexd is the relative velocity of the moving object,
xcext is the wavelength of the transmitted signal,
ƒt is the frequency of the transmitted signal,
c is the magnitude of the velocity of electromagnetic energy propagating in surrounding medium (free-space in this case) and is equal to the product of frequency and wavelength.
In certain embodiments of the invention, such a resulting Doppler signal, which modulates the oscillator signal, is detected by filtering it out of the incoming signal, amplifying it, filtering it again and converting it to a digital signal, preferably using a zero-crossing detector (ZCD). The output of the ZCD is ideally a square wave having a frequency that is the Doppler frequency. The detected digitized Doppler frequency signal is applied to the input port of a microprocessor, which measures the time between negative-going zero-crossings using an internal timer. The measurement of zero-crossing intervals are compared to certain criteria to verify that a valid signal is being processed. Then a Doppler frequency value is calculated from the measured zero-crossing information by taking the time between zero-crossings in the same direction as is equal to the period of the Doppler frequency. Using the above formula, the velocity of the moving object toward the sensor, for example, the speed of a thrown ball approaching the sensor, is then calculated. The calculated velocity magnitude is displayed on a small liquid crystal display (LCD).
The radar unit of the invention may be located in close proximity to the path of the object whose speed is being measured. It may, for example, be located such that the object moves within one or a few feet of the speed measuring device. This arrangement may place the radar unit within a few inches of the object whose speed is being measured for at least a portion of the flight of the object and within a few feet of the object for long enough for substantially all of the speed measurement to be made. For measuring projectiles from shooting implements, the radar unit is positioned so that the object is moving almost directly away from the unit with the speed of the object being measured within close proximity to the unit. In certain embodiments of the invention, the antenna of the unit may positioned in or very close to the path of the moving object with a signal processing portion of the unit positioned remote from the antenna and connected to the antenna by a transmission line. The antenna is typically of a fixed length and, if remote from the other circuitry of the unit, may be connected to the circuitry with a coaxial, parallel conductor or other transmission line that is impedance matched and designed into the RF detector circuitry.
By so locating the speed measuring radar unit immediately adjacent the path of the object whose speed is being measured and providing the unit with a short range of effectiveness of less than ten feet, and preferably of from one to three feet, velocity errors due to off-line location are minimized, since the Doppler frequency represents the velocity of the object in a direction toward or away from the radar unit. Mounting the radar unit on a gun barrel or archery bow, for example, places the unit in an effective location. With such placement, transmitter output power can be in the order of microwatts, which is much less than the radiated power levels of most wireless consumer products such as cellular and portable telephones. Short range detection avoids false readings of speed due to the motions or movement of the launching implement, a target or other item that might be in the field of view of the radar antenna.
The display may be positioned on the unit itself facing rearwardly so that the shooter or other user can read the output upon launching the object. The unit can alternatively provide an output signal that may be transmitted, through cable or a wireless link, to a remote display or other device. Mounted on a gun barrel or archery bow, the antenna portion of the radar unit may face the target while the display is oriented on the back of the radar unit so it is visible to the shooter or may be located elsewhere. An LCD, a battery and a power supply are located in the unit, with switches located on the unit and accessible to the user. The unit may also include a real time display such as that of a conventional digital wristwatch, which can share the battery and power circuit with the speed measuring device and utilize the display of the device to display time of day or elapsed time.
The radar velocity sensor can be operated from a 2.5 VDC battery power supply, requiring an average current of less than one milliampere. Thus, a single 3 volt nominal lithium cell capable of 160 milliampere-hours can power the sensor for a relatively long duration. Small, inexpensive cylindrical and button configuration lithium cells with this energy capability are readily available and are widely used in consumer products. Power xe2x80x9cON/OFFxe2x80x9d and xe2x80x9cResetxe2x80x9d switches are provided which are easily operated by the one hand of the user.
The velocity measurement device of the present invention is capable of being miniaturized and produced inexpensively so that it can be used in consumer applications, which, up to now, have not heretofore been addressed by the prior art. It can be built into, or attached to, a baseball or softball glove, to measure the speed of the ball being caught. The radar can be worn on the person of the user or on a launching implement. A radar unit can be built directly into the implement.
According to certain applications of the present invention, speed measurement of other sports objects is provided in applications where small portable devices may be used. For example, paint ball guns used in survival games and training use air pressure to propel paint balls or markers at other players. To avoid injury to the players being shot with the markers, the velocity of the markers at the barrels of the guns is limited to, for example, 300 feet per second. To optimize the trajectory of the markers, it is desirable to calibrate the guns so that the marker is as close to the upper velocity limit without going over the limit. One embodiment of the invention contemplates the fixing of a speed measuring unit or the antenna thereof on the barrel of the marker gun closely adjacent the barrel with the antenna aimed parallel to the barrel and the path of the marker. The device is adjusted to process Doppler readings for a speed range of, for example, 150 to 450 feet per second. To accommodate such speeds, band pass filter and clock speed settings are made to differ from those used for baseballs, etc. Depending on the anticipated speed of the object being measured, such settings should be made to exclude signals below and above the anticipated speed range to eliminate erroneous readings, and the timing should be such that a series of speed readings are made of the speed of the object traveling in the range of the signal.
Further, in archery competition, the trajectory of an arrow depends on precise control of the speed of the arrow leaving the bow. As in the paint ball application, the speed measuring device can be attached to a bow to measure the speed of an arrow leaving the bow. The device is preferably fixed to an extension forward of the bow, closely adjacent the path of the arrow. For example, the device may be fixed ahead of the tip of the arrow when the bow is drawn and at about or slightly ahead of the midpoint of the arrow when the rear of the arrow is resting against the undrawn bowstring. The device may, accordingly, be fixed on the end of a counterbalance bar that is fixed to and extends forward of the bow.
As with paint ball guns, firearms may be provided with the speed measuring device of the invention to measure the velocity of a bullet leaving the barrel of the gun. Parameters of the speed measuring device, for such an application, would be set to accommodate object speeds of from about several hundred to a few thousand feet per second.
These and other objectives and advantages of the present invention will be more readily apparent from the following detailed description of the illustrated embodiments of the invention, in which: